A transformer is a known electrical device widely used for transferring energy of an alternating current in the primary winding to that in one or more secondary windings. It typically contains two or more electrical circuits comprising primary and secondary windings, each made of a multi-turn coil of electrical conductors with one or more magnetic cores coupling the coils by transferring a magnetic flux therebetween.
Presently known three-phase transformers usually utilize E+1 magnetic cores in a flat structure. Such a transformer includes several interconnected magnetic cores located in one plane. U.S. Pat. Nos. 4,893,400 and 5,398,402 disclose transformers having a magnetic core made of an amorphous metal strip wound into a core over a mandrel, with one leg of the resulting core being subsequently cut off and with forming the metal into a rectangular shape. This transformer is manufactured in the following manner. A piece of rectangular steel is wrapped around the outer periphery of the amorphous metal core. The amorphous metal is then annealed, and the core is encapsulated in a resinous coating, except the cut leg. This allows the opening of the cut leg. The layers of amorphous alloy strips of the two edges are oriented so that the edges define top and bottom surfaces, each surface having a discontinuity defining a distributed gap portion extending from the top surface to the bottom surface. The coils are placed over two long legs and the cut leg is closed. The joint is then sealed.
According to U.S. '400, the sealing is made wit glass cloth and an ultraviolet-curable resin to provide the structure by the “fit and cure” method. This method is costly and labor-intensive. The transformers having amorphous metal cores manufactured according to this method cannot be repaired without causing damage to the core.
According to U.S. '402, the sealing is made with a porous material such as woven cotton cloth or paper. The porous material is folded over the joint and secured into position. An additional piece of porous material is placed through the window of the core, wrapped around the core and secured there. Electrical grade steel is disposed around the transformer core and is closed around the core joint and tack-welded. This structure allows the cut leg to be opened to permit replacement of a defective coil. The operation, however, is time-consuming and labor-intensive.
U.S. Pat. No. 5,441,783 discloses a technique of the kind specified, wherein a coating used to impregnate the core joint is a porous material with a viscosity greater than about 100,000 cps and a bonding material with a viscosity of at least about 100,000 cps. The porous material comprises strands of fiber, and the bonding material is thixotropic epoxy. Although the coated cores have good magnet properties, their manufacture requires costly and complex operational steps. Moreover, the method of repairing these cores is labor-intensive.
Another common disadvantage of the transformers manufactured according to the techniques disclosed in the above patents is that annealed amorphous metals become extremely brittle, and thus break under mechanical stress, for example, during the stage of closing the core joint.
In the transformers of the above kind, a planar core structure is used. U.S. Pat. No. 4,639,705 discloses a transformer structure of another kind, having a spatial magnetic core system. This structure has advantages over the planar “E+1” structure, such as the reduced quantity of required magnetic materials (by about 20-30%), reduced volume of the transformer, reduced core losses (by about 20-30%), and balanced currents in the three phases of the primary windings. However, to manufacture a transformer in accordance with the technique disclosed in U.S. '705, complex production technology as well as a complex repair technology, are required.
U.S. Pat. No. 4,639,705 discloses a transformer structure of another kind, having a spatial magnetic core system. This structure has advantages over the planar “E+1” structure, such as the reduced quantity of required magnetic materials (by about 20-30%), reduced volume of the transformer, reduced core losses (by about 20-30%), and balanced currents in the three phases of the primary windings. However, to manufacture a transformer in accordance with the technique disclosed in U.S. '705, complex production technology as well as a complex repair technology, are required.
However, the commercially available amorphous ribbons (strips) are typically limited in width (up to 200 mm). Thus, the width of the strip is typically much smaller than that required for the height of a transformer core.
It is known from the disclosure in U.S. Pat. No. 2,909,742 that in order to obtain a desired height of the transformer core, a number of toroids can be stacked on top of each other. This technique, however, suffers from energy losses caused by unavoidable introduction of unwanted air gaps between each two adjacent toroids.
GB Patent No. 1,164,288 discloses a technique of fabricating a cylindrical magnetic core for a power transformer. The magnetic core is made from coiled strips, wherein the core is of greater axial dimension than the width of the strip. To manufacture the core, a plurality of layers of the magnetic steel strips is simultaneously coiled to form the cylindrical core. The sum of the width of the strips in each layer is equal to the axial dimension of the core, and at least one longitudinal edge of each strip is staggered in relation to those in adjacent turns of the resultant coil.